The invention relates generally to articles including protective coatings for thermal barrier coatings. More particularly, the invention relates to articles including protective coatings for thermal barrier coatings, such that the protective coatings are calcium-magnesium-aluminum-silicon-oxide (CMAS)-reactive.
Thermal barrier coatings are typically used in articles that operate at or are exposed to high temperatures. Aviation turbines and land-based turbines, for example, may include one or more components protected by the thermal barrier coatings. Under normal conditions of operation, thermal-barrier coated components may be susceptible to various types of damage, including erosion, oxidation, and attack from environmental contaminants.
For turbine components, environmental contaminant compositions of particular concern are those containing oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof. These oxides combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-silicon-oxide systems (Ca—Mg—Al—SiO), hereafter referred to as “CMAS.” At the high turbine operating temperatures, these environmental contaminants can adhere to the heated or hot thermal barrier coating surface, and thus cause damage to the thermal barrier coating. For example, CMAS can form compositions that are liquid or molten at the operating temperatures of the turbines. The molten CMAS composition can dissolve the thermal barrier coating, or can infiltrate its porous structure by infiltrating the pores, channels or other cavities in the coating. Upon cooling, the infiltrated CMAS composition solidifies and reduces the coating strain tolerance, thus initiating and propagating cracks that may cause delamination and spalling of the coating material. This may further result in partial or complete loss of the thermal protection provided to the underlying metal substrate of the part or component. Further, spallation of the thermal barrier coating may create hot spots in the metal substrate leading to premature component failure. Premature component failure can lead to unscheduled maintenance as well as parts replacement resulting in reduced performance, and increased operating and servicing costs.
Thus, there is a need for improved coating systems that provide protection to thermal barrier coatings from the adverse effects of environmental contaminants, when operated at or exposed to high temperatures. In particular, there is a need for improved coating systems that provide protection to thermal barrier coatings from the adverse effects of deposited CMAS.